This application is based on application No. 11-358419 filed in Japan, the contents of which are incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal composition and a liquid crystal light modulating device such as a liquid crystal display which uses the liquid crystal composition.
2. Description of Related Art
In recent years, various kinds of reflective type liquid crystal displays which use chiral nematic liquid crystal have been developed. The chiral nematic liquid crystal is prepared by adding a chiral agent to nematic liquid crystal so as to exhibit a cholesteric phase at room temperature. Such a liquid crystal display makes a display by switching the liquid crystal between a planar state (colored state) and a focal-conic state (transparent state) when a pulse signal of a specified voltage is applied to the liquid crystal. The liquid crystal display can be so structured as to keep the colored/transparent states even after stoppage of the application of voltage.
This kind of liquid crystal is expected to be used in a different field from liquid crystal for displaying motion pictures. This kind of liquid crystal is typically used as a substitution for paper on which an image is printed. For example, the liquid crystal can be used for electronic blackboards, bulletin boards, electronic books, decorative panels, etc.
This kind of liquid crystal is desired to have the following characteristics: being driven by a low voltage; displaying an image at high definition with high contrast; having a wide operating temperature range; responding to a low temperature, etc. The liquid crystal is especially desired to be driven by a low voltage, to have a high response speed and to display an image with high contrast. There is, however, a well-known fact that when a chiral agent is added to nematic liquid crystal, the driving voltage becomes higher, and the response speed is lowered. Also, when some measures are taken to improve the liquid crystal in one or some of the above desirable characteristics, the liquid crystal is likely to degrade in other characteristics.
An object of the present invention is to provide a liquid crystal composition and a liquid crystal light modulating device which can be driven by a low voltage, have a high response speed and displays an image with high contrast.
In order to attain the object, a liquid crystal composition according to the present invention comprises: a nematic liquid crystal mixture which contains a liquid crystalline difluorostilbene compound; and a chiral agent. The liquid crystal composition exhibits a cholesteric phase at room temperature.
When a plurality of voltages of mutually different strengths are applied to such a liquid crystal composition which exhibits a cholesteric phase at room temperature, the liquid crystal composition switches between a state to selectively reflect light of a specified wavelength (selective reflection state) and a state to transmit incident light (transparent state).
Liquid crystalline difluorostilbene compounds generally have low viscosity and high anisotropy of refractive index. Therefore, by containing such a liquid crystalline difluorostilbene compound in such a liquid crystal composition, the viscosity of the liquid crystal composition becomes lower, the response speed of the liquid crystal composition becomes higher, and the driving voltage to drive the liquid crystal composition becomes lower. Also, the contrast which is the ratio of the Y value in the selective reflection state to the Y value in the transparent state becomes higher.
Liquid crystalline difluorostilbene compounds represented by the following formula are usable: 
R1: alkyl group with 1 to 10 carbon atoms or alkenyl group with 2 to 20 carbon atoms
A1: single bond or 1,4-cyclohexylene group
X1: xe2x80x94F, xe2x80x94Cl or xe2x80x94CN
Q1: xe2x80x94F or xe2x80x94H
Z1: xe2x80x94F or xe2x80x94H
The concentration of the liquid crystalline difluorostilbene compound in the liquid crystal composition is preferably in a range from 5 wt % to 60 wt %.
The liquid crystal composition according to the present invention may comprise other liquid crystalline compounds. For example, various kinds of liquid crystalline compounds generally used for nematic liquid crystal, such as liquid crystalline phenylcyclohexane compounds, liquid crystalline tolane compounds, liquid crystalline ester compounds, liquid crystalline pyrimidine compounds, liquid crystalline polycyclic compounds, nonpolar liquid crystalline compounds, etc., may be contained.
Such liquid crystalline difluorostilbene compounds can be produced by a conventional method. For example, the method disclosed by Japanese Patent Laid Open Publication No. 11-49707 can be adopted.
The concentration of the chiral agent in the liquid crystal composition is preferably in a range from 10 wt % to 45 wt %. By setting the concentration of the chiral agent in this range, deposition of crystal can be prevented, and the wavelength of light to be reflected by the liquid crystal composition can be set within a desired wavelength range.
The liquid crystal composition may further comprise a coloring agent. By adding a coloring agent, the color purity of the liquid crystal composition can be adjusted.
A liquid crystal light modulating device according to the present invention comprises: a pair of substrates, at least one of which is transparent; and a liquid crystal layer which is sandwiched between the substrates, the liquid crystal layer comprising the above-described liquid crystal composition. The liquid crystal light modulating device requires a low driving voltage, has a high response speed and can display an image with high contrast.
The liquid crystal layer may have a memory effect. In this case, even after stopping application of a voltage to the light modulating device, the light modulating device can keep displaying an image. Therefore, the liquid crystal modulating device can be used, for example, as an energy-saving display device.
On the substrates, electrodes are formed, and further, an insulating film which function as an alignment controlling layer and/or a color filter may be provided thereon. The insulating film stabilizes the operation of the liquid crystal light modulating device and improves the light modulation performance.
Between the substrates, polymer nodules may be provided. The polymer nodules keep the gap between the substrates more accurately and contributes to achievement of desired light modulation performance.
The substrate which is at a light incidence side may have an ultraviolet-ray absorbing characteristic or an ultraviolet-ray absorbing layer. Also, the other substrate which is opposite the light incidence side may have a light absorbing characteristic or a light absorbing layer.